#686 Support for subscription preapproval

RFC 6121 §3.4 outlines how a user can pre-approve subscription requests for a given JID. This feature makes "pairing" between XMPP IM users more seamless and allows for faster networking between users.
To make the XMPP experience more modern/easy/$buzzword, it would be great to have support for pre-approval in prosody.
This is one part of the https://wiki.xmpp.org/web/Easy_Roster_Invitations strategy.